Electrical energy storage devices, particularly batteries, which employ fibrous carbon or graphite electrodes and which operate in a non-aqueous electrolyte at ambient temperature are known from U.S. Pat. No. 4,865,931, issued Sep. 12, 1989 to F. P. McCullough et al, the subject matter of which is in its entirety incorporated herein by reference. The patent generally discloses a secondary battery comprising a housing having at least one cell positioned in the housing, each cell comprising a pair of electrodes of an electrically conductive carbonaceous material, a foraminous electrode separator for electrically insulating the electrodes from contact with each other, and an electrolyte comprising an ionizable salt in a non-aqueous fluid in each cell. The electrodes are characterized as carbonaceous electrodes having a Young's modulus of greater than 1 MM psi (6.9 GPa), but less than 55 MM psi (380 GPa).
A similar electrical storage device is disclosed in U.S. Pat. No. 4,830,938 to F. P. McCullough et al, issued May 16, 1989, the subject matter of which is incorporated herein in its entirety. This patent discloses a fibrous carbonaceous electrode which is characterized as having a Young's modulus of greater than 1 MM psi (6.9 GPa), but less than 75 MM psi (517 GPa), most preferably from 20 MM to 45 MM psi (138 GPa to 311 GPa), and a surface area with respect to the fibrous material of at least 0.1 m.sup.2 /g, most preferably less than 5 m.sup.2 /g. The patent additionally discloses a shared (bipolar) carbonaceous electrode which is capable of carrying a current from one cell to an adjacent cell without a current collector frame associated therewith and when employed as the electrode in a series of adjacent cells of a battery having but one terminal electrode provided with a collector frame at each end of the battery.
Although the batteries disclosed in the prior patents to McCullough et al provide a major advance in the battery arts, the electrodes require a special graphite material such as THORNEL.RTM. fibers (mesophase pitch based graphite fibers made by Amoco) which have a Young's modulus of from 45 to 55 MM psi, or PANEX.RTM. PWB-6 fibers (stabilized acrylic based carbon fibers made By Stackpole) which have a Young's modulus of from 33 to 38 MM psi. It is stated in the prior patent to McCullough et al that electrodes made from RPG graphite (Nuclear Grade Reinforced Pyrolitic Graphite) having a Young's modulus of less than 1 MM psi, or electrodes made from GRAFOIL.RTM. (a compressed graphite foil manufactured by Ucar Carbon Co. Inc., while having the required degree of carbonization, electrical conductivity, and surface area, do not have the required physical properties of Young's modulus and aspect ratio and thus have failed when used as the positive electrode in non-aqueous battery systems due to destructive intercalation of large anions from the electrolyte between the graphite layers.
Destructive intercalation of large anions between the carbon or graphite layers of an electrode and electrolyte causes instability which becomes a particular problem in electrical energy storage devices, particularly rechargeable batteries that operate with a non-aqueous electrolyte and having a cell voltage of greater than 2.5 volts. Such batteries require exacting standards during their manufacture and operation to prevent the introduction of gaseous water or water vapor into the batteries since the introduction of only minute quantities of water into a battery results in the electrolysis of the water molecule. Electrolysis of water molecules can take place during electrical charging of the battery at a potential of greater than 2.5 V, during storage in the charged state and during discharge of the battery, forming O, OH- and H+ ions with the OH- ions migrating to the positively charged cathode where electrolysis to O and O.sub.2 takes place. The OH- and H+ cause a breakdown of such commonly used non-aqueous electrolyte materials such as propylene carbonate through catalysis, while highly reactive O and O.sub.2 destroy the surface of the carbon or graphite electrode due to destructive intercalation. McCullough et al report that their battery is capable of operating with a water content of up to 300 parts per million (ppm) but that it will have a somewhat reduced cycle life. McCullough et al also report that if the water content should become onerous, that the battery can be disassembled, dried and reassembled in a dried state without substantial damage to its continued operation.